Sony SDXC cards work in NX5U, but not AX-2000

I'm using a Sony AX-2000 as a second cam on a shoot with an NX5U. I bought two 64GB Sony SDXC cards recently. They work great in the NX5U, but the AX-2000 keeps shutting off and back on, like the software is restarting. I assume it is because it is not recognizing the SDXC format, but the specs for the camera say it can indeed use these cards. Given the similarities between the two cams, I'm surprised by the issue.

I have a note into tech support, but I might not hear back from them in time. Any ideas?

Did you use the in-camera format media function? It's usually best for the card to be formatted by the camera it will be used in, and this should be done before each use. Obviously you'll lose everything on the card so back it up to your computer first, each card copied into its own folder. Just be sure not to keep old clips on your cards and delete individual clips to make room. The NX5's even have a hidden "Media Empty" function that I use periodically because even with formatting the cards before every shoot, eventually I'll lose a couple minutes of maximum record time. At that point it's time to do a Media Empty.

Also, I prefer 32GB SD cards maximum, be it for NX5s for work or GoPro H3+ for play. Seems like people tend to have problems with 64GBers, maybe it's all in my head.

Unfortunately, they can't be formatted. There's an error message about the card being full and unable to record HD. If you try to format, that's when the camera acts as if it is re-booting and shuts off. In the NX5U, they worked right out of the package. Sometimes instead it gives an error "This function is currently not available."